The LTE (Long Term Evolution) Advanced under the 3GPP (Third Generation Partnership Project) has proposed OFDMA (Orthogonal Frequency Division Multiplexing Access) using MU-MIMO (multi-user multiple-input multiple-output). In the downlink transmission of the MU-MIMO, one base station can not only communicate with a plurality of mobile communication terminals but can also simultaneously transmit different data streams (layers) to one mobile communication terminal.
The LTE Advanced group has been discussing a reception technology for mobile communication terminals, called interference rejection combining (Interference Rejection Combining, IRC). The interference rejection combining is a technology of weighting, at a mobile communication terminal, each signal obtained by the respective reception antennas so that the mobile communication terminal reduces the interference of interfering radio wave beams from interfering base stations to desired radio wave beams from a visited base station (desired base station) in the downlink communication.
The IRC enhances the reception quality of desired radio wave beams of a desired signal especially when, as shown in FIG. 1, a mobile communication terminal 10 is located near the boundary of a visited cell area 1a (the cell area of a desired base station 1) and is subject to strong interference radio wave beams from a different base station 2 (interfering base station) that is next to the desired base station 1. In FIG. 1, the reference symbol 2a denotes the cell area of the interfering base station 2. FIG. 1 depicts a schematic shape of a beam 1b generated at the desired base station 1 and that of a beam 2b generated at the interfering base station 2 also. The beam 2b generated at the interfering base station 2, in other words, a part of beams for a downlink channel to another mobile communication terminal (e.g., mobile communication terminal 12) is a cause of an interfering signal 2c interfering the mobile communication terminal 10.
Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2 are example documents describing the IRC.
Non-Patent Document 2 discloses a reception weight matrix calculation of the IRC, using the MMSE (Minimum Mean Square Error) algorithm. This technology can calculate a reception weight matrix (interference rejection combining reception weight matrix) WIRC(k, l) of a mobile communication terminal receiving a signal in accordance with the following Equation (1) derived from the MMSE algorithm.WIRC(k,l)=H1H(k,l)R−1  (1)
In Equation (1), H1 is a channel impulse matrix of a signal from a desired base station (desired cell) to which the mobile communication terminal connects; R is a variance-covariance matrix of a received signal vector for the mobile communication terminal. The letter k denotes a reception subcarrier number (i.e., a subcarrier index), and the letter l denotes an OFDM symbol number (i.e., an OFDM symbol index). The superscript H on the right-hand side of Equation (1) denotes a complex conjugate transpose.
A variance-covariance matrix for the received signal vector can be calculated from a channel impulse matrix obtained from a vector representing a reference signal received from the desired base station. A calculation method for a variance-covariance matrix based on a vector representing a reference signal is publically known. A variance-covariance matrix for a received signal vector can be calculated from a vector representing a data signal. A calculation method for a variance-covariance matrix based on a vector representing a data signal is publically known.
The 3GPP categorizes reference signals (reference signals, RSs) into cell-specific reference signals (cell-specific RSs (CRS)), channel state information reference signals (channel state information RSs (CSI-RS)), and demodulation reference signals (demodulation RSs (DM-RS)). The demodulation reference signals are also called terminal-specific reference signals (UE-specific RSs).
The 3GPP-LTE, i.e., Release 8, requires the use of cell-specific reference signals (CRS). A cell-specific reference signal supports a maximum of 4 transmission antennas of a base station (cell). The cell-specific reference signal is used for determination of channel state information (channel state estimation), demodulation of data, measurement of signal reception qualities (RSRP, reference signal reception power) from cells, and demodulation of control channels (Dedicated Physical Control Channel, DPCCH).
The 3GPP-LTE-Advanced, i.e., Release 10 LTE and after, would require channel state information reference signals (CSI-RS) and demodulation reference signals (DM-RS). A channel state information reference signal supports a maximum of 8 transmission antennas of a base station (cell). The channel state information reference signal will be used only for the determination of channel state information (channel state estimation). Accordingly, in comparison with the cell-specific reference signal, the channel state information reference signal is transmitted at lower density (with longer intervals).
A demodulation reference signal supports a maximum of 8 transmission streams that can be transmitted from a base station (cell). The demodulation reference signals would be used for demodulating mobile communication terminal (User Equipment, UE)-specific data signals. The demodulation reference signals have been precoded in substantially the same way as for data signals, and therefore, mobile communication terminals are able to demodulate data signals using the demodulation reference signals without any precoding information.
The LTE Advanced possibly uses also cell-specific reference signals, for example, for reception quality (RSRP) measurement of signals from cells and for control channel demodulation. FIG. 2 shows an example mapping of various signals transmitted in a single resource block of an LTE Advanced downlink transmission. The reference symbol RB denotes one resource block, and each segment of the resource block shows a resource element RE consisting of one subcarrier and one OFDM symbol, which are the minimum resource unit. The vertical axis shows frequency (subcarriers), and the horizontal axis shows time (OFDM symbols).
According to the LTE Advanced, mobile communication terminals determine channel state information (channel state information, CSI) from SINRs (signal-to-interference-plus-noise ratios) and feed the determined channel state information back to a base station. In response to the channel state information fed back from the mobile communication terminals, the base station executes precoding for forming transmission beams suited for respective mobile communication terminals. The precoding alleviates interferences from a plurality of transmission beams that use the same frequency, to improve reception qualities at a plurality of mobile communication terminals that receive these transmission beams. Known as an example of the channel state information fed back from mobile communication terminals is a set of a channel quality indicator (CQI), a precoding matrix indicator (PMI), and a rank indicator (RI).